An example of using a Punnett square for blood typing would be predicting a child's possible genotypes based on the blood types of his parents. The filled-in Punnett squares would show all the possible genetic combinations based on the mother's and father's potential genotypes, according to Arizona State University.
If a woman with type AB blood and a man with type B blood mated, a Punnett square could be used to determine the child's potential allele combinations for blood type, reports Arizona State University. Because the A and B alleles are codominant, someone with type AB blood must have one of each allele. This means the mother in this example must have the AB genotype. In someone with type B blood, there are two possible combinations: BB and BO. It would take two Punnett squares to show all the potential allele combinations for this couple's offspring.
The first Punnett square would show the possible combinations for a parent with the AB genotype and a parent with the BB genotype. The potential combinations would be AB and BB, states Arizona State University. The second Punnett square would show the possible combinations for a parent with the AB genotype and a parent with the BO genotype. In this case, there are four possible combinations: AB, BB, AO and BO. Because O is recessive, it would not be possible for the child to have type O blood. Based on these Punnett squares, this couple's offspring would have either type AB or type B blood.